The present invention relates to a new and improved television audio receiver system and is more particularly directed to apparatus and a method for receiving and reproducing stereo television sound.
Under present television broadcasting standards, a band of frequencies approximately 80 KHz wide is designated within each 6 MHz television channel for the transmission of the audio component of a television signal. Within this band of frequencies, an RF main audio carrier signal is frequency modulated by an audio baseband signal for producing a main aural audio transmission signal. The transmitted main aural audio signal is received by a television receiver that converts the RF audio carrier signal to a signal having a frequency centered at 4.5 MHz. The converted 4.5 MHz sound carrier is then processed by an FM detector to reproduce the main aural audio signal that was used to frequency modulate the RF audio carrier at the transmitter.
The Federal Communications Commission has recently approved a standard for broadcasting stereophonic television sound. The standard approved uses the Zenith Broadcast Delivery System and the dbx noise-reduction system. This system was, in part, an outgrowth of the well-known techniques for transmitting stereophonic audio signals which has been popular in radio broadcasting for some time. The basic FCC-approved system for stereophonic radio broadcasting is disclosed in U.S. Pat. No. 3,257,511 to R. Adler, et al. In this system, the arithmetic sum of left (L) and right (R) audio source signals (L+R), commonly referred to as the main channel modulation, is used to directly frequency modulate the RF carrier signal. The difference between the left and right stereophonically related signals (L-R) is used to amplitude modulate a 38 KHz subcarrier signal in a suppressed carrier fashion with the resultant double-sideband signal being impressed as frequency modulation on the radiated RF carrier. In addition, a pilot subcarrier signal of 19 KHz is transmitted for synchronization of the FM receiver. The FM receiver extracts the 19 KHz pilot subcarrier, doubles its frequency, and applies the resulting 38 KHz signal to a synchronous detector where the (L-R) difference signal is recovered from the amplitude modulated 38 KHz stereophonic subcarrier. The recovered (L-R) modulation is then suitably matrixed with the (L+R) main channel modulation in order to recover the original left and right stereophonic signals.
The foregoing stereophonic radio broadcasting system often also includes an SCA component which allows broadcasters to provide a subscription background music service. The SCA component comprises a 67 KHz subcarrier frequency modulated by the background channel program, the frequency modulated subcarrier being used to frequency modulate the main RF carrier signal together with the stereophonic modulation.
Various systems and apparatus have been proposed for the transmission of stereophonic sound together with a conventional television picture transmission. These systems normally utilize the radio broadcasting stereophonic transmission techniques discussed above but with, in most cases, different subcarrier frequencies selected for their compatibility with the transmitted video signal. One such prior art system is disclosed in U.S. Pat. No. 4,048,654 to Wegner. This patent discloses a transmission system in which a composite baseband signal identical to that employed in FM stereophonic radio broadcasting is employed to frequency modulate the main sound carrier of a television transmission signal. Thus, the proposed composite baseband signal includes an (L+R) main channel component, an amplitude modulated double-sideband suppressed-carrier 38 KHz subcarrier (L-R) component and a 19 KHz pilot component. In another embodiment, the use of a subcarrier signal having a frequency (f.sub.H) characterizing the transmitted video signal is proposed in lieu of the 38 KHz (L-R) channel subcarrier to reduce interference from the video component of the television signal.
Another system, which was proposed in U.S. Pat. No. 3,099,707 to R. B. Dome, also employed the conventional stereophonic radio broadcasting system but with an (L-R) channel subcarrier equal to 1.5f.sub.H and a pilot signal equal to 2.5f.sub.H. These frequencies were selected to minimize the effect of the video components of the television signal appearing in the recovered sidebands of the (L-R) channel signal.
U.S. Pat. No. 3,046,329 to Reesor discloses yet another similar system in which the composite baseband signal used to frequency modulate the main sound carrier includes only the main channel (L+R) component and the upper sidebands of the (L-R) channel signal amplitude modulated on a subcarrier having a frequency of 2f.sub.H. Other prior art system for stereophonic television sound transmission have proposed the use of frequency modulated subcarriers for the (L-R) stereo channel typically centered at 2f.sub.H, although a center frequency of 1.5f.sub.H has also been proposed.
As previously mentioned, in addition to transmitting stereophonic sound components on the main aural carrier of a transmitted television signal, it is also desirable to transmit additional information thereby more completely exercising the available audio bandwidth within a television channel. For example, the transmission of a second audio program ("SAP") signal would enable a viewer to selectively operate a television receiver for reproducing the audio signals associated with the transmitted stereophonic information, or alternatively, the audio signals associated with the transmitted second audio program which may comprise, e.g., a foreign language version of the television program.
One prior art proposal for providing a second language capability in connection with a transmitted television signal is disclosed in previously mentioned U.S. Pat. No. 4,048,654 to Wegner in which the two channels of a stereophonic-like signal are employed. In particular, the (L+R) main channel signal is used to transmit a first language audio signal and the (L-R) stereo channel signal is used to transmit a second language audio signal. U.S. Pat. No. 3,221,098 to Feldman discloses a transmission system allowing for the simultaneous broadcast of a single television program having up to four or more different language soundtracks by forming a composite baseband signal consisting of four or more different subcarrier signals each amplitude modulated with a different language audio signal, the composite baseband signal being used to frequency modulate the main RF audio carrier. Yet another proposed second language system uses a frequency modulated subcarrier baseband signal centered at 2f.sub.H for both stereophonic sound transmission and for second language transmission. A pilot signal, modulated with one of two different frequencies, is used to indicate which service is being broadcast.
The foregoing systems and techniques for transmitting different audio signals in conjunction with a standard television transmission were not adopted in the U.S. for a number of reasons including, in certain cases, poor performance and, in others, incompatibility with U.S. television transmission standards.
The concept behind the Zenith stereo broadcast system adopted in the U.S. is disclosed in U.S. Pat. No. 4,405,944 to Eilers et al. This system comprises an audio transmission system that is fully compatible with U.S. television broadcasting standards and is capable of providing stereophonic sound transmission together with a second audio program service.
In the Zenith stereo broadcast delivery system, audio information is located in the region from about 4.4 to 4.6 MHz above the video carrier of a television channel allocation. The audio portion takes up only about 0.20 MHz, which is small compared to the large portion of bandwidth occupied by the video (luminance and chroma) signal. In the past, a monophonic audio channel was transmitted as an (L+R) FM signal with a frequency range of 50-15,000 Hz. In the Zenith system, a pilot signal has been added at the horizontal scanning line frequency f.sub.H (15.734 kHz) to allow new stereo receivers to locate a second channel for stereo, which resides from 16.47-46.47 kHz (centered at 2f.sub.H) from the bottom of the audio allocation. This second channel is the key to receiving stereo sound, as it is an (L-R) AM signal with the same frequency range as the mono channel. Stereo is achieved when the L-R and L+R signals are combined.
A third channel, the second audio program or "SAP", is provided in the Zenith system for bilingual programming and other commentary. The SAP channel is FM and extends from about 65 to 95 kHz (centered at 5f.sub.H) with a frequency range of 50 Hz to 12 kHz. Professional channels which may be used for voice or data can be inserted into the remaining audio space of about 98.2 kHz to 106.5 kHz (centered at 6.5f.sub.H). Several types of sound channel processing for these audio signals at the home television receiver are known.
One such processing technique is provided by a "separate aural carrier receiver", in which the aural carrier is processed separate from the visual carrier. Since the aural carrier is transmitted without incidental phase modulation ("ICPM"), none can reach the FM detector so that this receiver can be free of all video related buzz.
A second known receiver for television sound channel processing is referred to as the "split sound receiver". This method of sound processing was used in the early days of television before intercarrier detection was introduced. The video and sound portions of a received television signal are down converted to a lower frequency and the sound component of the composite signal is pulled off and processed to provide an audio output. In the split sound receiver technique, tuner-introduced ICPM can cause low frequency noise in the sound output.
Neither separate aural carrier receiver techniques nor split sound receiver techniques can be used in a cable television environment due to the high FM noise in the oscillators used to down convert the television signal. Expensive oscillators with separate tuning systems would be required to overcome this problem, and thus the techniques are not economically viable in cable television systems.
A third known type of sound channel processing is referred to as the "quasi-split sound receiver". In this technique, separate processing of the sound and video signals is used, but with synchronous detection combined with intercarrier sound detection. Such a receiver is disclosed in U.S Pat. No. 4,405,944 referred to above. Nyquist ICPM is eliminated in the quasi-split sound receiver by a specially designed IF filter with symmetrical response centered at the video carrier. Although this type of receiver is relatively immune to tuner-introduced ICPM, microphonics, local oscillator phase noise, reverse mixer feedthrough to the tuner of local oscillator, and to video related frequency modulation caused by the AFC/AFT circuits, it suffers from distortion caused by interfering harmonics of the television horizontal line frequency. Such harmonics fall within the pilot signal, the (L-R) subchannel and the SAP signals.
It would be advantageous to provide an apparatus and method for receiving stereo broadcast television sound which avoids such interference. Such an apparatus and method should be able to be used in the cable television environment and remain uneffected by harmonics of the television horizontal line frequency, as well as phase noise due to jitter in the cable television converter and local oscillator tuning loop.
The present invention provides an apparatus and method with these advantages, through the use of two separate receivers for the (L+R) signal and the pilot, (L-R), and SAP signals. The result is a substantially improved quality of television stereo sound reception.